The present invention generally relates to solid oxide fuel cells and, more particularly, to a method of fabricating a solid oxide fuel cell electrode with engineered structures to improve performance characteristics.
A fuel cell is basically a galvanic conversion device that electrochemically reacts a fuel with an oxidant within catalytic confines to generate a direct current. A fuel cell typically includes a cathode material which defines a passageway for the oxidant and an anode material which defines a passageway for the fuel. An electrolyte is sandwiched between and separates the cathode and anode materials. An individual electrochemical cell usually generates a relatively small voltage. Thus, to achieve higher voltages that are practically useful, the individual electrochemical cells are connected together in series to form a stack. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
The fuel and oxidant fluids are typically gases and are continuously passed through separate cell passageways. Electrochemical conversion occurs at or near the three-phase boundary of the electrodes (cathode and anode) and electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.
Specifically, in a solid oxide fuel cell (SOFC), the fuel reacts with oxide ions on the anode to produce electrons and water, the latter of which is removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxide ions that diffuse through the electrolyte to the anode. The electrons flow from the anode through an external circuit and then to the cathode, with the circuit being closed internally by the transport of oxide ions through the electrolyte.
In a SOFC, the electrolyte is in solid form. Typically, the electrolyte is made of a nonmetallic ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons, which ensures that the electrons must pass through the external circuit to do useful work. As such, the electrolyte provides a voltage buildup on opposite sides of the electrolyte, while isolating the fuel and oxidant gases from one another. The anode and cathode are generally porous, with the anode oftentimes being made of nickel/YSZ cermet and the cathode oftentimes being made of doped lanthanum manganite. In the solid oxide fuel cell, hydrogen or a hydrocarbon is commonly used as the fuel and oxygen or air is used as the oxidant.
Various methods have been employed for making the four materials of a fuel cell, such as that shown in U.S. Pat. No. 4,476,198. Therein, the compositions used for the four materials are put into four distinct slurries. Each slurry is then placed in a reservoir of a squeegee-type device which is pulled over a flat surface and hardens or plasticizes into a layer of the material having the desired thickness. In this manner, an electrolyte wall or interconnect wall is formed by a first layer of anode material followed by a layer of either electrolyte or interconnect material and finally by a layer of the cathode material. The layers are capable of being bonded together since the binder system is the same in each layer.
U.S. Pat. No. 4,816,036 shows the compositions of the four materials being individually mixed with a binder and a plasticizer to form a plastic mix, and each then being processed by hot-rolling into a tape form. The tapes are combined in a trilayer form as trilayer electrolyte walls and trilayer interconnect walls. The walls are then assembled, stacked, and sintered to form a monolithic solid oxide fuel cell.
Another example of forming a monolithic solid oxide fuel cell is U.S. Pat. No. 5,162,167. Ceramic powders for each of the four materials are provided. The powders are mixed with a desired binder and plasticizer in a high intensity mixer. The mixed materials are formed into respective tapes by a roll mill. The individual tapes are then used to form green state multilayer tapes comprising an anode-electrolyte-cathode or an anode-interconnect-cathode. The green state multilayer tapes can then be cut and molded into desired net shape elements which can be assembled into pairs. After the paired elements are densified, they are stacked and bonded with a bonding material. The stacked elements are then sintered to provide a monolithic assembly. A method similar to the above is shown in U.S. Pat. No. 5,256,499.
As seen in U.S. Pat. Nos. 5,286,322; 5,256,499; and 5,162,167, the ceramic powder often used in making the anode comprises a mixture of yttria-stabilized zirconia and nickel oxide. It is known that the addition of zirconia reduces the thermal expansion of the anode that otherwise occurs during operation of the fuel cell, with temperatures frequently being in the range of 700 to 1,000 degrees C. Unless controlled by a component such as zirconia, the thermal expansion and contraction of the anode produce thermal stresses that can lead to failure of the anode and components associated with it.
On the other hand, it is known that the mixing of zirconia with nickel oxide tends to create a random positioning of the nickel atoms, which leaves a less than optimum condition for electrical connectivity. The less than optimum condition is created because the random mixing of the components tends to cause the nickel atoms to migrate towards one another at high operating temperatures and thereby create voids in the already random arrangement of nickel atoms. These voids tend to reduce the electron flow in the anode and, accordingly, reduce the effectiveness of the fuel cell. To compensate for the reduced effectiveness of the nickel, the concentration of the nickel can be increased. But then the thermal expansion of the anode increases, resulting in increased thermal stresses. Also, the overall cost of the fuel cell may increase.
Yet another disadvantage of random mixing of the zirconia and nickel oxide is the limited control over thermal expansion of the anode. With random mixing, the concentration of the nickel must typically be at least 30 volume percent to provide sufficient conductivity to the anode. This required concentration sets the low limit of thermal expansion that can be controlled for the anode. The random mixing in the prior art also limits control in terms of the fuel uptake or access by the anode. Since fuel access is being governed by a random placement of nickel throughout the anode, the degree of fuel access in any one point in the anode is essentially a random circumstance.
As can be seen, there is a need for an improved method of making a SOFC. There is also a need for an improved electrode and, particularly, an anode, as well as method of making the same for a SOFC. A need exists for an improved anode and method of making the same which minimizes thermal expansion while maximizes electrical connectivity. In particular, there is a need for an anode and method of making the same which minimizes the randomness of the anode components to maximize the electrical connectivity. Also needed is an improved anode and method of making the same which provides the ability to tailor thermal expansion and fuel access.